The present invention pertains to the use of sputtering as a means for filling trenches and vias with copper (and alloys thereof). Sputtering techniques include Gamma copper (xe2x80x9clong throwxe2x80x9d deposition), IMP copper (ionized metal plasma deposition), coherent copper, and traditional (standard) copper deposition. In particular, when the sputtering of copper is carried out under particular process conditions, it is possible to fill feature sizes of 0.4 micron or less having aspect ratios of 1 or greater.
As the feature size of semiconductor patterned metal features has become increasingly finer, it is particularly difficult to use the techniques known in the art to provide multilevel metallurgy processing. In addition, future technological requirements include a switch from the currently preferred metallurgy of aluminum to copper, because of copper""s lower resistivity and higher electromigration resistance. The standard reactive ion etching method frequently used for patterning a blanket metal cannot be practiced with copper, since there are no volatile decomposition products of copper at low temperatures (less than about 200xc2x0 C.). The alternative deposition liftoff techniques are also limited in applicability in a copper structure, given the susceptibility of copper to corrosion by the lift-off solvents. Therefore a damascene structure is used which requires the filling of embedded trenches and/or vias.
A typical process for producing a multilevel structure having feature sizes in the range of 0.5 micron (xcexcm) or less would include: blanket deposition of a dielectric material; patterning of the dielectric material to form openings; deposition of a conductive material onto the substrate in sufficient thickness to fill the openings; and removal of excessive conductive material from the substrate surface using a chemical, mechanical, or combined chemical-mechanical polishing techniques. Currently the conductive material is deposited using chemical vapor deposition (CVD), evaporation, and sputtering. Chemical vapor deposition, being completely conformal in nature, tends to create voids in the center of the filled opening, particularly in the instance of high aspect ratio features. Further, contaminants from the deposition source are frequently found in the deposited conductive material. Evaporation is successful in covering shallow features, but is generally not practical for the filling of high aspect ratio features. Sputtered copper, prior to the present invention, was not considered as a technique for filling of high aspect ratio openings, as voids typically occurred along the sidewalls of the openings. The sputtering technique included cold (typically below about 150xc2x0 C.) deposition of sputtered copper so that the copper would adhere to the substrate surface, followed by an annealing process (without deposition) at temperatures in excess of about 400xc2x0 C., to reflow the copper and obtain filling of the trench or via. However, such a reflow process takes hours, due to the low bulk diffusivity of copper.
U.S. Pat. No. 5,246,885 to Braren et al., issued Sep. 21, 1993, describes the problems listed above, and proposes the use of a laser ablation system for the filling of high aspect ratio features. Alloys, graded layers, and pure metals are deposited by ablating targets comprising more than one material using a beam of energy to strike the target at a particular angle. The ablated material is said to create a plasma composed primarily of ions of the ablated material, where the plasma is translated with high directionality toward a surface on which the material is to be deposited. The preferred source of the beam of energy is a UV laser. The heating of the deposition surface is limited to the total energy deposited by the beam, which is said to be minimal.
U.S. Pat. No. 5,312,509 of Rudolph Eschbach, issued May 17, 1974, discloses a manufacturing system for low temperature chemical vapor deposition of high purity metals. In particular, a semiconductor substrate including etched patterns is plasma cleaned; subsequently, the substrate is coated with adhesion and nucleation seed layers. A reactor connected to the process chamber containing the substrate sublimes a precursor of the metal to be deposited, which is then transported to the substrate. A reactor heat transfer system provides selective reactor cooling and heating above and below the precursor sublimation temperature under the control of programmable software. The heated chuck on which the substrate sits heats the substrate above the dissociation temperature of the precursor, releasing the metal from the precursor onto the substrate to nucleate the metal species onto the seed layer on the substrate. Then the system is pumped to a lower pressure and the substrate is advanced to the next process chamber. This manufacturing system is recommended for the chemical vapor deposition of pure copper at low temperatures. Although an adhesion barrier layer (and a sputtered seed layer if required) are said to be deposited using sputter deposition, the copper layer is applied solely by CVD deposition, to avoid the sidewall voiding which is said to occur if sputtering is used for the copper deposition. The CVD copper deposition is carried out using a wafer temperature controlled within a temperature range of 120xc2x0 C. to 250xc2x0 C. during the nucleation of the metal species upon the substrate (with the temperature being lower at other times during the process).
U.S. Pat. No. 5,354,712 to Ho et al., issued Oct. 11, 1994, describes a method for forming interconnect structures for integrated circuits. Preferably, a barrier layer of a conductive material which forms a seed layer for metal deposition is provided selectively on the sidewalls and bottom of interconnect trenches defined in a dielectric layer. Subsequently, a conformal layer of metal is selectively deposited on the barrier layer within the interconnect trench. The metal layer comprises copper which is deposited by chemical vapor deposition from an organo-metallic precursor at temperatures. In particular, the layer of copper is deposited by CVD from copper (hexafluoroacetylacetonate) trimethyl vinylsilane compound by pyrolysis at low temperatures, between about 120xc2x0 C. and 400xc2x0 C., onto a conductive barrier layer of sputtered titanium nitride (TiN) which lines via holes, providing a seed layer for selective growth of the conformal layer of copper. The temperature of the substrate surface on which the conductive barrier layer resides does not appear to be specified.
In any case, this process suffers from the conformal deposition of the metallic layer which tends to cause voids in the center of the filled opening, as previously described, and from the presence of contaminant residues from the precursor material which remain in the deposited metallic fill.
U.S. Pat. No. 5,585,673, issued to Joshi et al. on Dec. 17, 1996, discloses refractory metal capped low resistivity metal conductor lines and vias. In particular, the low resistivity metal is deposited using physical vapor deposition (e.g., evaporation or collimated sputtering), followed by chemical vapor deposition (CVD) of a refractory metal cap. Recommended interconnect metals include Alx,Cuy (wherein the sum of x and y is equal to one and both x and y are greater than or equal to zero). The equipment required for collimated sputtering is generally difficult to maintain and difficult to control, since there is a constant build up of sputtered material on the collimator over time. Collimated sputtering is described in U.S. Pat. No. 5,478,455 to Actor et al., issued Dec. 26, 1995. Collimation, whether for sputtering or evaporation, is inherently a slow deposition process, due to the reduction in sputtered flux reaching the substrate.
It would be highly desirable to have a sputtering process for copper deposition which uses a substantially standard sputtering process chamber and target, while providing a complete fill of vias and trenches.
It has been discovered that the surface diffusion characteristics of copper over a particular temperature range enable the complete filling of vias and trenches using sputtering techniques previously believed incapable of achieving such filling.
In particular, the copper fill layer may be applied in a single step process or in a two step process. In the single step process, for feature sizes of about 0.75 xcexcm or less, when the aspect ratio of the feature to be filled is less than approximately 3:1, the temperature of the substrate to which the copper fill layer is applied should range from about 200xc2x0 C. to about 600xc2x0 C. (preferably from about 200xc2x0 C. to about 500xc2x0 C.); when the aspect ratio is about 3:1 or greater, the copper fill layer should be applied over a temperature ranging from about 200xc2x0 C. to about 600xc2x0 C. (preferably from about 300xc2x0 C. to about 500xc2x0 C.). The deposition can be initiated at the low end of the temperature range, with the temperature being increased during deposition.
In the two step process, a thin, continuous wetting (bonding) layer of copper is applied at a substrate surface temperature of about 20xc2x0 C. to about 250xc2x0 C. The wetting layer thickness (on the wall of the trench or via) should be a minimum of about 5 nm, and typically may be about 10 nm to about 30 nm, depending on feature size and aspect ratio. Subsequently, the temperature of the substrate is increased, with the application of fill copper beginning at about 200xc2x0 C. or higher and continuing as the temperature is increased to that appropriate for the feature size. When both the copper wetting layer and the copper fill layer are applied in a single process chamber, the deposition may be a continuous deposition. In such case, process conditions are varied during the deposition, with the copper fill layer being applied at a slower rate than the copper wetting layer, to provide better deposition control.
When the copper wetting layer is applied in one process chamber and the copper fill layer is applied in a second process chamber, typically the substrate with copper wetting layer already applied is placed on a heated support platen in the second process chamber. For a small feature size (0.5 xcexcm or less) and an aspect ratio of 1:1 or greater, it is better to wait until the substrate is heated to a temperature of at least 200xc2x0 C. prior to beginning application of the copper fill layer, or to begin the fill layer deposition at a slower rate while the substrate is heating.
The selection of a single step process or a two step process depends on the composition and structure of the surface upon which the copper is being deposited and the feature size of the trench or via to be filled.
The copper sputtering technique used in the single step process is selected from Gamma deposited copper, Coherent copper, IMP copper, and traditional standard sputter deposition copper.
The copper deposition method used for application of the thin, continuous, wetting layer of copper in the two step process may be one of the sputtered copper techniques listed above or may be chemical vapor deposition (CVD) copper or evaporation deposited copper, depending on the feature size of the trench or via to be filled. The deposition method used for the copper fill layer is selected from the sputtering techniques listed above, to provide a more contaminant-free and more rapid filling of the trench or via.